class TestContains:
  def __contains__(self, arg):
    return arg

print(0 in TestContains())
print(1 in TestContains())
print('' in TestContains())
print(0 not in TestContains())
print(1 not in TestContains())
print('' not in TestContains())

CPython:

False
True
False
True
False
True

because https://docs.python.org/3/reference/expressions.html#membership-test-operations

For user-defined classes which define the contains() method, x in y returns True if y.contains(x) returns a true value, and False otherwise.

MicroPython:

0
1

True
False
True

Unexpected results so definitely worthy of a cpydiff addition, or should this be fixed?